The use of portable battery operated equipment or systems that employ complex high performance electronic circuitry has increased recently with the widespread use of, for example, cellular telephones, laptop computers, and other systems. In such systems, it has become important to provide a voltage reference. Likewise, in such a system, it is desirable for this voltage reference to operate at a relatively low power supply voltage, such as on the order of 3 volts, and it is desirable that the voltage reference be stable and substantially immune to temperature variations, power supply variations, and noise.
Typically, a circuit known as a bandgap voltage reference generator is employed to provide the desired stable reference or bandgap voltage. One such bandgap voltage reference generator is described in U.S. Pat. No. 4,849,684, entitled "CMOS Bandgap Voltage Reference Apparatus and Method," by Sonntag et at., issued Jul. 18, 1989, assigned to the assignee of the present invention and herein incorporated by reference. Such a bandgap reference is particularly useful for a variety of applications; however, typically the bandgap voltage reference described in the aforementioned patent requires a power supply on the order of about 4 volts to produce a voltage reference or bandgap voltage reference of about 1.25 volts. Other bandgap voltage reference generators are also described elsewhere, such as the bandgap voltage reference generator described on pages 295-296 of Analysis and Design of Analog Integrated Circuits, by P. Gray and R. Meyer, 2d. Ed., available from John Wiley & Sons, and shown in FIG. 4.30c of that publication, herein incorporated by reference. Such a bandgap has disadvantages including the fact that the bipolar transistors in the circuit may not be realized in a standard MOS integrated circuit fabrication process because the collectors must be floating. Likewise, instead of producing a bandgap voltage reference that is substantially independent of temperature, it may be desirable, in some circumstances, instead to have a current source that produces a current substantially proportional to absolute temperature. Such a current source may be employed to provide a bandgap voltage reference, while also providing greater flexibility with respect to alternate applications. Unfortunately, prior art bandgap voltage reference generators typically do not provide a PTAT current in a manner so that the current is available for circuitry external to the generator. Thus, a need exists for a current source that is capable of providing a current substantially proportional to absolute temperature that will operate satisfactorily with a relatively low voltage power supply, such as below 4 volts.